Power generating and superheating method, and apparatus therefor



W. H..ROWAND June 24 1958 POWER GENERATING AND SUPERHEATING 2840054 METHOD. AND APPARATUS THEREFOR 3 Sheets-Sheet 1 Y Filed Oct. 8, 1951 www "Nm www.

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POWER GENERATING AND SUPERHEATING METHOD, AND APPARATUS THEREFOR Filed Oct. 8, 1951 5 Sheets-Sheet 2 2.1" ff'zi29 .aaa Hnl IHm nid Ms! hhlllml nin nin m n n ni MA um lln Hm hill mnl h|a||L 90 I I lNvEN'roR 12 jowand BY ma United States Patent O POWERGENERATING AND SUPERHEATING METHOD, AND APPARATUS THEREFOR Will H. Rowand, Short Hills, N. J., assignor to The Babcock & Wilcox Company, Rockleigh, N. L, a corporation of New Jersey Application October 8, 1951, Serial No. 250,268

49 claims. (C1. 122-479) This invention relates to a method of vapor generation and superheating, and a vapor generating and superheating unit of the once-throughy type for carrying out the method. V. ,Y

More specifically, the inventiony relates to improvements-in4 superheated steam generators of the oncethrough type in which the operating medium is forced under pressure into` one end of an externally heated tubular passage or passages and is delivered from `the outlet endor ends of the passage or passages ,as highly superheated vapor. g

The invention also involves a method of operationof sucha steam `or vapor generator, vthe method including the limiting of the liquid to vapor transition zone to the convection sectionby the coordination of load, furnace tiring rate and furnace temperature with the variable proportioning of furnace and convection section absorption of the total available heat in counter-acting relationA to the tendency ofthe transition zoneto move toward or from the furnace under the influence of load changes.

It is .an object ofthe inventionjto provide a forcedflow tubular steam generator having 4a transitionV zone wherein the position at which the working uid is com-t;

pletely converted from the. liquid to the vapor state located ina portion of the heating space of such relatively low heat intensity that injury to the tubing of the transition zone will be minimized, even though such tubing may accumulate chemical deposits at the transition position, together with means responsive to characteristics of the working uid to control the distribution of heat application-between the radiant vaporizing and convection superheating surfaces in such a manner as to prevent substantial shifting of the transition position within the tubing of the transition zone, particularly under .variable yload or slagging furnace conditions. n

The-invention minimizes load variation the transition position in a forced-flow vapor generating andsuperheating unit having radiantly heated liquid containing tubes in the 4combustion chamber wallsandk serially connected convection heatedtubes'` comprising the transition -zone including the transition position.

locatedv in a heating space receiving gasesfrom the combustion chambenby controlling the proportion of heatk absorbed by the furnace tubes as compared with the heat'y chamber,introducing,,recirculated `cooled heating gases to onefor moreselelcted positions infthey furnace. l, f

paratus wherebyuhe-selective regulation ofrecirculated gas introduction is controlledby means;responsive to the shifting of 2,840,054 Patented June 24, 1958 ICC 2 temperature of the partially superheated vapor and by means responsive to combustion Y chamber temperatures, while the supply of combustion media is controlled jointly from the flow and temperature of the delivered superheated vapor.

The pertinent vapor generating and superheating unit of this invention involves a section including one or more tubes into the inlet ends of which a vaporizable liquid is pumped at high pressure. these tubes, the vaporization of the liquid is completedr and the vapor is superheated. There is thus a transition zone within these tubes and it has been found that the variations involved in the temperatures of the gases heating such tubes cause corrosion of the tube metal because of chemicaldeposits from the vaporizable liquid.

Attempts have been Vmade to remedy the above indicated difculty,'such attempts involving a re-routing of the tubes ofthe transition zone relative tok that zone Within this tube, or i and the furnace zone from which the gases pass to the, former. Such attempts have not been successful because'.

they have limited the effective load range of the pertinent units and have thus seriously limited the commercial,y

application of units of this type. They have also undesirably involved low feed water temperatures.

This invention involves the recirculation of furnace gases from a position downstream gas flow-wise of the evaporating'wall surface of the furnace to a point where feed water temperatures can be raised to desirably high:l values and to ypermit a wide load range on the unit while 1 still keeping the transition zone-inya position lof such- Y lowered gas temperature that the difficulties arising from chemical deposits and tube metal corrosion in the transi-v` tion'zone are minimized.

The invention increases the overall thermal eciency yof the pertinent type of vapor generating and superheating t unit and makes it particularly applicable for operation at fluid pressures varying from 2000 p.v s. i. to 5000 p. s. i.

The invention will be described with reference to the accompanying drawings in which the transition zone is disposed in a convection gas pass receiving gases from a furnace' section, the Walls of which include tubes in whichY turn bend tubular sections extending transverselyof gas ow in the convection section. Thus, the pertinent part of the convection section involves longpor continuousconduits into the inlet ends of which a vaporizable liquid vis pumped at high pressures. Within these conduitsthe'I Vcompletion of the vaporization is accomplished and they' vapor is superheated. From the outlets of these tubes', the superheated vapor passes to a bankvoftubes for` further superheating in a high temperature superheating| zone, these tubes being disposed transverselyof the,

furnace gasesr flowing from the furnace section` ytothe transition zone. I

The `burners for the furnace zone are-disposed between Y, two locations` at either vor both of which recirculated gases., from a position in .the gas stream downstream of: they' transition zone; are controllably introducedto properly` vary and control the-heat absorption of thewall-tubesf;

of the furnace zone and control the gas temperatureatg;

Y; the furnace outlet. Y f

-In the udrawings:

Fig. 1 is a diagrammatic view of a control system for the illustrative vapor generating unit;

Fig. 2 is a diagram illustratingthe forced flow of fluid throughthe various components of the unit; andv Figfis a'n enlarged vertical section through the illustrative .vapor `generating and superheating unit. a

If .a oncethrough boiler is considered as consisting of a single length of tubing with water introduced at one end an'dYh'eat applied along the tube so that superheated stejarisV delivered fromf the other end,A we Vhave a situation in which atfsjome localized position the `final and complete vaporizationof the uid takes place. This is a place-of initial dryness w itlr100% steam quality andis variously referredjto s theconversion or transition position. Ahead offthis position, ewfroin the position of watery introductior'l'p touthetfransition position, the waterris being progressively heate'rl up toffsatur'ation temperature and then progressively vaporizedup to fthe transition fposition. From the transition` position `tol the steamro'utlet` or delivery p'oit, in `so far asrthie tubingis subjected to heating gases'of sufficient temperature,` there is an increasein heat content ofthe steam with an increase of temperature or degree 'of superheat. -o o a A 1If sucha boilerisoperatedrwith a variable rate of feed water introduction, that "is, if 'thefeed rate is raised or lowered, with a Corresponding modification of heat input in order to obtain proper thermalelciency, then the posi-V tion ofrthelconversionor transition position will shift,fpar ticuularlyrin those installations where a portion of the vaporization heat.` input isI 'accomplished by furnace wall tubes. The transition positionrwill shift toward the feed inlet end of the ytube with reductions infeed ow and toward the superheater outlet end' with increases in feed.

`The transition positionand the lengthv of `tubing or zone `throughwhich this positionl moves Iwith varying loads, has

beensub'jec't to the deposition of chemical salts `from the water as` theiresult of evaporationt'o dryness. When this tr'ansi'tionpoint and'thezone is' placed ina hightemperature position, for -example, in the'furnace,4 this salt deposit so reduces the heat transfer that the tubing becomes over# heated andfails. lt'hasbeenproposed to take'the transitionlzone out"` of the high temperature heat absorbing position'by 'placingfit backinthei'convection pass so that even though.interrrahaccumulation"of salts did occurfthc heat transfer would'be so low th'at'overheating would not occurduring the operatingperiod. t The salts wouldthen be removedfby washing or otherwise. Y t v The'particular fault VofY such` anfarrangem'ent is'that inasmuch as the conversionpositio'n s'not xed within the`tube butshiftsthroughaconsiderable length of tubing when operated over variablezloadsnzit'involves a consid--i` Y erable'length of tubing which must be frequently cleaned and given better protection'against failure'by overheating andV corrosion by 'the` us'eof :theproper alloy steel.

invention, minimizes Vtheshifting 'of the transition zone, thereby reducingi'installation costs byv minimizing.

' through 4forced)circulation boilers, and tlejvo'lu'mes of the furnaces are'si so'as tojinsuieIl agas: 'temperature iirgjthe amount "off-heat 'absorbed by the furnace walls is'such'a great proportion of the total"necess`a`ry Vto evaporate'the water ndsuperheat the]stear`n,that there Iis little evaporation to be accomplishedfin 4a't'ransition p g thefurnaces'whn hfvvilljbe sufcientlyglowto avoid, slggitig difficulties, particularly Yin Ithe ease rof r'l'aulv'erized zone if it is moved into a relatively cool gas pass. If the amount of heating surface located in the gas pass is of considerable extent the reduction of load might not shift the transition position back into the tubes of the furnace lining, but this would not be feasible if an advantageous range of load variation is to be accomplished.

The invention has two phases-both of which are related to gas recirculation. The one phase is that involv ing gas recirculation to affect furnace heat absorption to the end that the transition position shifting is minimized. This is by the introduction of the recirculated gas to thc bottom of the furnace. The second phase involves the utilization of gas recirculation for the high load range, introducing it into the upper portion of the furnace as close to the furnace gas outlet and convection pass inlet as possible when considering the matter of gas mixing, so that the gas temperature at the entrance of the convection pass may be minimized irrespective of the rate of heat liberation byfuel burning. 'f

Fig. l of the accompanying drawings involves `a verti- This vertically elongated 4furnace 10 provides a radiation zone or lfurnacel zone inwhich the vaporizable fluid is The high temperaturevaporizable `liquit'r'l'tloi/vs from the lower headers: -20faiidf22`=th`rough th'ewalltub'es of the furnace and to 'such rtubular connections Las f those indi` cated at 24 and 26, leadina vidingr a secondary convectionuid hcatin'gizonef This lower header is indicatedin its entirety by :the numeral 36.

pass walls, the'uid owsthrough the 4'w2`1ll"tubes1538 to -a superposed component 40 of a similarlyU-shaped oppositely arranged upper-gas Apass headerindicatedin-its entirety at 42 (Fig. `2). Similarly, fluid liows from the component. 32 *of the lower=headerf36"through the side wall tubes' 44 to..a component'46 of`the` upper /gaspass header.` From the rear wall-component 33`of the lower header'36 the fluid flows upwardly through wall tubes l48,

vthe upper parts of which havefroof'itubciscctions 50 directly leading tothecomponent 52 ofthe upper gaspass header. From this header,` the fluid tlows Vto theinlet header 58 of the transition section. Thisftra'nsition section 4is formedjfby rsuperposed `banks 604-63 of serially connected return.' bend tubes, leading to outlettl'icader fromwhich thesupcrhcated vapor ows through the con duitV 68` to an-atte1uperator 70 "and thence=to the inlet header '72 of thesuperhcater S. f Here, the vapor is Vfurther.superheated-to'highcr temperatures within conillustrated trait.' Fig; A2 digrmmneuy aiseises the' econ'onizer E into the tubes" of :whichlr'liquid is fortzedat high pressure by the' punpjsn. @Frein -thefeesnemizer-buto'w takes 'place "t gh the `conneclet headerljflu I p v 4 I to' the `Aheader sectioni 2'2 'and 41an tors suchfas'BZl an of the furnace subho'pper 88.

From the small header or header section 2'2", the fluid passes upwardly through the furnace wall tubes extending along the right hand side of the combustion chamber 10, as it is shown in Fig. 1. This wall may be also referred to as kthe rear wall of the furnace. These tubes have their lower parts 90 extending along the wall of the subhopper 88 and succeeding parts 92 upwardly inclined to-V ward the right and extending along the right hand wall 94 of the hopper 96, thence the tube parts 98 extend along the division wall between the gas pass 100 and the combustion chamber. Above these parts the tubes have other parts 102 associated with appropriate heat resisting material 104 to form the wallV of a gas pass across the rear side of the furnace arch 106, the upper part of which forms the lower wall 113 of thefurnace or combustion chamber gas outlet in which the superheater S is located. The upper wall or" the arch is defined by the tube parts 110 (Fig. 3) together with associated and suitable heat resisting material 112, and the lower part of this arch is dened by the tube parts 114 and associated heat resis-ting material 116. At the rear of the superheater S the parts 118 and 120 of the vapor generating tubes form a screen through which the gases pass to a gas turning space 122 at the top of the transition section and the gas pass 100.

'The pertinent vapor generating tubes extend as roof tubes continue to the outlet header 28.

Referring back to Fig. 2, the lower headers for the furnace or radiation section R may be formed as small units such as 130-134 leading to the right of the inlet header section 22, or the headers may be diaphragmed to provide such small header sections. Similarly, successive small header secti-ons 136 and 138 are arranged at the left of the inlet connection 82. The header arrangement at the top of the furnace section includes similar small header sections 141-147.

The fluid flow from thereconomizer E of Fig. 2 through the header 18 and the connector 84 continuesfthrough the header section 22 and thence through connected wall tubes 23 upwardly to the header section 14S. It continues from this header section through one or moredowncomers 150 to the next lower header section 130. From the latter, the ow is upwardly through connected ywall tubes 23 to the header section 146. From this header the ow continues through succeeding wall tubes 25 and downcomers to the upper'header section -28 from which the fluid flows through lines 24 and 27 to the header coming zone G.

Similar flow continues from the economizer outlet header 18 through the connector 82 and header section 86.

connected wall tubes 29 discharging fluid to the upper header section 144. From this header the uid flows downwardly through the downcomer 150 to the succeeding lower header section 136, the ow continuing in this manner around the walls of the furnace section to theV upper header section 30. rFrom this header section the iiuid flows through a connector 26 to the component 34 of the lower header of the secondary convection fluid heating zone involving the walls ofthe gas pass 100.

Flow continues upwardly through the wall tubes of the secondary convection uid heating zone or the gas pass wall section to the upper U-shaped header 42 for the gas pass. From the components of this header, uid ows through the lines or conduits 1752-154 to the inlet header 58 of the transition section T. rThe flow 4continues through the serially connected tubes of this section to the outlet header 66, the conduit 68, the atternperator 70 and the inlet header 72 of the superheater S. From From this header section the flow takes place through the outlet header 78 the superheated vapor iows toa point of use. Y

Referring further to the diagrammatic disclosure of Fig. 2, it is to be understood that the disclosure of the radiation zone structure at R indicates the entry of fluid into two adjacent header sections 22 and 86 in the midpart ofthe right hand or rear wall (Fig. vl) of the cornbustion chamber. The vsucceeding header sections such as 145, 146, 147, -132 to the right of the line 84 represent the upper and lower header sections for onehalf of the rear wall of the combustion'chamber, then the corresponding header sections for one side wall, the last few upper and lower header sections at the n'ght hand part of Fig. 2 indicating the upper and lower headerv sections of one-half of the front wall of the combustion chamber. Similarly, the successive upper and lower header'sections to theleft of the line 82 represent first the remainder of the header sections for the rear wall, then the header sections for the other side wall, and beyond that the corresponding header sections for the remainder of the front wall of the combustion chamber. In other words, the component parts of Fig. 2 represent the walls of the combustion chamber folded into a single plane which might be considered the plane of the rear wall of the combustion chamber.

The invention involves the maintenance of the zone of transition from a liquid-vapor mixture to completely vaporized condition at some position along the oncethrough tubes extending from the inlet header 58 to the outlet header 66 of the transition section, and a'temperature control point such as T is so selected that it is at a minimum distance above or beyond the Vinlet header S8 only suiciently that under all load variations (and attendant factor variations), the transition zone is always above the lower header 58 of the transition section.

To maintain the transition zone within the stated limitations, flue gas recirculation, either Ato the hopper throat (through a plurality of ports distributed along V the throat) or through a plurality of wall openings such 40' i right duct 182. The gas is forced by a fan 184 through a horizontal'duct 186, to a junction box 188 and theny through a duct extension in which a control `damper 192 is positioned. From the junction box, controlled ilow of recirculated gas also takes place through upright 4 duct 194 and then through transverse duct 178. Flow through the duct 194 is controlled by a damper 196. Flow from the transverse duct 178 continues into the furnace through ports 172.

For automatic control of the illustrative systemk pressure responsive control element P is connected to the superheated steam line 200 leading from the outlet header 78 of the high temperature superheater. From this control element, such as a Bourdon tube, pressure impulsesk are communicated through a pneumatic pilot valve 203 (such as Vshown in U. S. patent to Johnson 2,054,464), a loading line 202, an averaging relay 210 (such as shown by U. S. Patent 2,098,913, device 13 of thaty patent), line 209, and motor controller 211 for an electric motor for driving the pump 80 for forcing waterinto the inlet header 12 of the economizer. The water. delivery of this pump is also controllable from load' variations, the load impulses being transmitted as repp resentativeof steam dow-water flow-ratio bythe pilot valve 205 and the line 206 from an orice device F (or steam flow controller) having tubular connections to opposite sides of the orifice 208,and measuring steam flow. Y The impulses from steam llow controller F are' modified by impulses from water flow controller W connected by lines 213 and 215 toopposite sides of the oritice 217. n For Pthese combined impulses the devices F and,` W are indicated as interconnected at 221. The ow metersor controllers F andW as well -as the recirculated A maybe interconnected by linkage to position a pilot valve (such as 205) in accordn with the relations between the two iluid flow rates. Further, Fig. 4'ofBirchler 2,526,843 shows another way Vof inter-relating steam flowand water ow where each of two meters establishes 1a fluidloading pressuregand then the two loading `pressures entera ratio gauge setting upa` resultanttluid pressure in pipe leading to averaging -relay A.` l`hesteam tlow,V or load line 206, joinsy the pressure `line 202, `at -the averaging -relay 210 by which combinedpressure impulsesand steam `fiowwaterlflowl-ratio impulses are transmitted Ato the motor controller 211 `for the variablespeed pump motor to control the operation of the water `pump 80 in desired or controlled ratios.` A f u m The operator 2-11 may involyea servo-motor such as that shownat `40 in VU. S. Patent 2,298,257 operating the movable member of a combined switch and `rheostat. The relay 210 iscapable of being so-adjustedthat either one of thepressure or loady impulses may have a predominating effect upon thefluid pressure in the line 209, and hence, upon the operation of the pump 80.

The steamow controller, or orifice device F measures steam flow,-or boiler load, and, associated with `one or more pilot valves, suchas 201etfectsrload variations in the loading lines 212, V216, and A'226.

Assuming that the fan1 84-is operated at a constant speed, the :dampers 192 `and 196 are so operated as to cause a maximum tlow of recirculated flue `gas into the hopper throat through the duct `190 at a low load. Under such conditions the minimum flow takes Vplace through the upright duct 194 and Vtherconnected upper recirculated,

gas openings or ports `17,2, the dampersbeing automatically controlled to accomplishthis result. This automatic control involves a temperature responsive element K, re

sponsive to gas temperatures at the outlet ofA- the furnace or, gas inlet of the high temperature superheater, the temperature indications 4of Vthis, element being transmitted through line V191fto Agas' ,temperature controller `193 `and then transmitted as, pressureloading impulses rthrough a pilot valve 14977. Theseimpulses are then transmitted from that valve `as corresponding f loading `variations throughn a line'212' -to totalizing relay 195, and thenf through'selectorvalve 199 and line-207 `to a-valve operator 214 forV the damper AThese control impulses are modified by steam 4flow; or load impulses transmitted from the control deviceF 'throughjpilot Avalve 201, line 21'2, andftheflinell to relay -195. .The relative-effects of the gas temperature'impulses and Vthe load impulses may be desirably regulated :for their effect upon the damper1'9'6a11`d thegasllowthroughthe upperports 172 so that one orV the otherof 'theimpulses may have apredominating ,eiect t I The flow ofrecirculated ue gas through the duct 190 to the hopper-throat170is controlledvtby intermediate steam VtemperatureV controller T 1responsive to `superheat temperatures 4(at Tf) in oneor more tubes beyond e the transition position.V AFrom this element and "its pilot valve 219, temperature indicationsare transmitted as ,cor'

210),one of the uid chambers of which is connected by the line226`to'the pilot valve `201 of the `steam flow measuring device, or's'teain' tlow controllerF. The relay 221' alsoreceivesfmodifyingtimpulses representative of changes j in,` the `jovv fof` recirculated, gases throughthe. duct 182. "For` thispurposdmthe"recirculated gasfwow:`

Fig.l 1 of amigarissasisaisoA Y portion of furnace will not efectasubstantial reduction in Y the'radiant absorption of thetfurnacewalls, but -onfthe' controller 252 is connectedby the lines 254 and 256 to opposite sides of ,the orifice 258. Changes in the pertinent gas flow are measured by the controller 252 and translated into representative loading impulses by the associated pilot valve 260, and its associated connections. `These impulses are then transmitted to relay 221 through line "262.

The control `of the operation ofthe burners 230 forv the combustion chamber is effected by control impulses transmitted to fuel and Vair control, or primary airdamper drive 240 (which may involve oneormore servo-motors such as shown in the Johnson Patent 2,536,184, January 2, 1951) in which the control Vimpulses. are selectively proportionately derived from the final steam temperature controller T3 connected to temperature responsive element 241 in the steamY line '200,and the steam flow controller, or load measuring device F. Fig. l of the Wheaton Patent 2,155,986 shows a steam temperature controller, which is representative of the steam temperature controllers T and Ti. The final steam vtemperature controller T3, connected to the temperature responsive element 241 by the line 270, controls the final temperature in the steam in the line 200, and through the associated pilot valve 272 transmits representative loading variations in the line 274 to the relay 276. The load impulses through the line 274 are controllably varied by this relay and are transmitted therefrom by the line 278 to one of the operative chambers of the totalizing relay V280. Y

The relay 280 also receives load impulses from the steam fiow controller F, through the line 216. Thus, Vthe loading impulses transmitted by the line 282 from the relay 280 Ato the selector valve 284 are the results of the combined effect of the load impulses in the lines 27S and 216 entering the relay 280.

The location of the transition position depends upon the amount ofheat absorbed in the furnace Wall generat-` In general, the volume of recirculated gas increases as the boiler load decreases to maintain the transition position within the desired limits. The illustrative control system is effective to 'vary the amount of tlue gas introduction into the'furnace and theposition of that introduction into the furnace from variables which include the tluid temperature at T. is on the superheated lsteam side ofthe transition zone.

The illustrative control system is elective to accom- Iplish the desired results by producing a maximum introduction of recirculatedV gases at low load through the duct 190 into the hopper throat of the installation, thefiow of recirculatedtue gas at this position being reduced to Zero as the load increases to a maximum ilow. Reversely, the maximum of recirculated ue gas takes place through the upper ports 172 at full load and is reduced toward zero at low load. Y

When the unit is operating at maximum steam output recirculated gas will be introduced into the upperpo-rts 172, thereby adding gas Aweight to the fresh products of combustion but dilutingthem so that the temperature leaving the furnace 'is below a predetermined maximum.

This introduction of `recirculatedgases to `the upper othery hand it does increase the gas weight owingr, over the ,convection surfaces. t

This control point a reduction in feed delivery from pump 80 and a reduction of fuel and combustion air input. The reduction in heat release in the furnace wifl drop the furnace exit gas temperature, permitting a reduction in the recirculated gas added through ports 172.

As the radiant heat absorption in the furnace walls will drop only slightly with 'the reduced heat rinput and not proportionately with the reduced water ow, the position in the tubes at which the fluid is completely evaporated in transition section T will move downward toward the inlet header 58. This will result in a rise in the slight degree of vapor superheat determined by temperature responsive element T.

When the temperature determined by T rises, it operates conjointly through 221 with a determination of the reduced steam output to effect an adjustment of damper 192 to regulate introduction ofrecirculated gas into the throat of the furnace.

Such introduction of recirculated gases into the bottom of the furnace will reduce the radiant absorption of heat by the furnace walls so that a greater weight of gases having a higher total heat content will be delivered from the furnace to the convection superheater and the transition section. This increase in gas weight is particularly effective in raising the proportion of the heat absorbed by the convection heating surfaces so as to adjust the proportion of heat absorbed in superheating to that absorbed in vaporization to the end that the location of the transition position will not shift widely, and particularly not back into the furnace, with the desired nal degree of superheat being concurrently maintained.

The invention involves the use of fiue gas recirculation in a once-through steam generator through either r both the hopper of the furnace or in the exit part of the furnace and this action takes place in such a controlled manner that not only the total amount of heat absorption by the evaporating wall surface of the furnace is reduced to a point where feed water temperatures of modern power plant practice can be satisfactorily uti-v lized, but permitting a wide load range on the unit while still maintaining the transition Zone in a cooled gas section of the steam generator, downstream gas flow-wise of the high temperature superheater. Low feed water temperatures to the boiler do not t in with modern regenerative turbine heat cycles which obtain high thermal efficiency through the use of progressive regenerative heating of the feed water to relatively high temperatures.

The gas weight increase due to recirculation willresult in a gas temperature leaving the economizer which will be slightly higher than the temperature which would occur for the same fuel input without the recirculation. While the increased gas flow entering at slightly vlower temperature may not appreciably affect the amount of heat absorbed by the superheater, which is the first surface passed over, its effect on increased absorption becomes more pronounced as it passes down over the evaporative surface below the transition position and then over the economizer. A slight decrease in total heat absorption by the convection surfaces will occur due to the increased gas weight and also slightly higher gas exit temperature (or stack temperature). This alone would have an effect of moving the transition position downward towards the inlet header 53 and correspondingly A increasing the extent of surface above the transition position which is superheating surface. Thus the superheat temperature might tend to be slightly deficient but it would be corrected by the controls regulating the fuel input. The controls, however, would also operate to bring this into proper balance following the regulation of the recirculation to the upper part of the furnace.

Inthe illustrative arrangement, the recirculated gases,v

which are introduced to prevent slagging temperatures,

are introduced at a level intermediate the height ofthe furnace. Their introduction will undoubtedly affect the the transition position upward from the inlet header 58.

The location of ports 172. at an intermediate position is a compromise measure for attaining mixing of the recirculated gases and products of combustion before they reach the superheater surface. An optimum position for introducing such recirculated gases would be directly at the furnace gas outlet if uniform mixing with the products of combustion coming directly from the burners could still be effected.

Although the invention has been described with reference to a specific unit, and its operation, it is to be appreciated that the invention is not limited to all of the details thereof, but is rather of a scope commensurate with the scope of the sub-joined claims.

I claim:

1. In a once-through vapor generating and superheating unit having a primary section including a furnace with liquid heating tubes along its walls, furnace firing means, said tubes receiving heat radiantly transmitted from the furnace gases, a liquid to vapor transition section of the once-through type including tubes receiving heated liquid at their inlet ends and heated by the gases after they have left the furnace to complete the vaporization within a transition zone flow-wise ahead of the outlet ends of the tubes, means forming a gas pass leading from the furnace and having the transition section arranged therein to receive all ofthe gases exiting from the furnace, a recirculated gas system extracting furnace gases from a position downstream Vgas flow-wise of the transition section and introducing the recirculated gases into the gases in the furnace at a position affecting the heat absorption of a major portion of the length of said furnace Wall tubes, and means operating to increase the flow of recirculated gases through said system when the transition zone tends to shift toward the furnace tubes and thereby preventing any substantial shifting of the transition zone in that direction.

2. In a once-through vapor generating and superheating unit having a radiation section including a furnace with liquid heating tubes along its walls, furnace firing means, said tubes receiving heat radiantly transmitted from the furnace gases, a liquid to superheated vapor transition connection section of the once-through type including long tubes receiving heated liquid at their inlet ends and convectionally heated by all of the gases from the furnace to complete the vaporization and initiate the superheating of the vapor Within a transition zone flow-wise ahead of their outlet ends, a superheater receiving the superheated vapor from the tubes of the transition Section and contacted by higher temperature furnace gases to further superheat the vapor, and means for maintaining the transition zone within the transition section including a recirculated gas system regulably withdrawing furnace gases from a position downstream gas flow-wise of the transition section and introducing the recirculated gases into the gases in the furnace partially ata position between the furnace firing means and the transition scction and partially aheadof the furnace firing means in a gas flow sense.

3. In a Vmethod of operating a once-through vapor generating and superheating unit having a furnace with liquid heating tubes therein; means firing the furnace, and a convection section receiving gases from the furnace and including a once-through bank of series connectedy vection section by controllably,recirculating gases from'a" superheated vapor temperature and vapor flow,r saidA regulating of recirculated gas flow tothe` second position involving the changing of the iiow inversely to the tendency` of the `vapor temperature to change.

4. In a method of operating a once-through vapor generating and superheating unit having a furnace with liquid heating tubes therein, means tiring the furnace, and a convection section receiving gases from the furnace and including a convection vapor superheater having foncethrough liquid to vapor transition tubes receiving liquid from the outlets `of the furnace-tubes, the Vmethod of Y regulating the division of the total available hcatof the unit between the furnace tubes andthe tubes ofthe convection section by controllably recirculating furnace gases from an inlet point `gas flow-wise beyond the furnace gasV outlet to first and second positions within the furnace to stabilize the zone of liquid to vapor transition in the convection section, pumping vaporizable liquid into the unit in accord with indications of vapor pressure and from indications of vapor flow from the unit, increasing the pumping rate when the pressure or vapor flow falls below a predetermined value, controlling the rate of recirculated gas flow to said iirst position (between the tiring means and the furnace gas outlet) from indications of gas 'temperature at the furnace outlet `and from indications of vapor flow from the unit, controlling the rate ofring from indications of vapor flow andlfrom indications of iinal superheated -vapor temperature, of the firing rate involving an increase in the firing rate whenrthe vapor tlow or the nal 4superheated vapor temperature falls Y of intermediate superheated vapor temperature adjacent the transition zone and from indications of superheated vapor flow from the unit and indications of recirculatedgas flow, the controlling ofthe rate of recirculated gas flow to said first `position involving the increasing offgas tiow as the gas temperature at the furnace outlet and the vapor llow tend to `increase while the'controlling of the recirculated gas flow at the second position involves increasing the gas flow as the intermediate superheated vapor temperature andthe superheated vapor flow tend to decrease.

5; ln a Vmethod of operating a vapor generating and superheating unithaving a furnace with liquid heating tubes thereirnineans tiring the furnace, and a convection section receiving gases from the furnace and including a convection vapor superheatcr, the method ofregulating the division of the total available heat of the unit between gas flow-wise beyond the furnace gas outlet 4to first and secondpositions within the furnace, controlling the rate of recirculated gas iiow to said first position (between the tiring means and the furnace gas outlet) from indica-V tions lof gas temperature at the vfurnace outlet and from indications of `vapor liow `from the unit, controlling the Y rate of tiring fromindications of vapor flow and from indications of nal superheated vapor temperature, the control` of the tiring rate involving an :increase 'in the (tiring. rate when the vapor flow'or thelinal ysuperl-teated vaporj temperature `falls `below a predeterminedfvalue, andcontrolling;the'jiiow of recirculated furnace gases` to the secondposition (upstream gas flow-wise of the firing means) from indications of intermediate superheated vapor temperature and from indications of superheated vapor ilow from the unit, the controlling of the rate of recirculated gas tlow to said tirst position involving the increasing of gas ow as the gas temperature at the furnace outlet and the vapor flow tend to increase while the controlling of the recirculated gas flow at the second position involves increasing the gas liow as the intermediate superheated vapor temperature and the superheated vapor flow tend to decrease. i

6. ln a vapor generating and superheating unit having a furnace provided with a heating gas outlet, liquid heating tubes associated with a boundary wall of said furnace adjacent the point of fuel entry and arranged to receive heat mainly by radiation from the heating gases in the furnace, fuel burning means normally burning a iluid fuel in suspension in the furnace at such a location as to provide heat radiation from Vthe main combustion zone to said furnace wall tubes, and a liquid to superheated vapor transition section including ,convection tubes arranged to receive heat from heating gases after .they have left said gas outlet, each of said convection tubes receiving at one end liquid from the generating tubes and discharging superheated vapor at its other end, the method of controlling the liquid to superheated vapor transition Zone and maintaining it in said transition section, which comprises withdrawing heating gases from a location in the heating gas flow downstream gas l'iow-wise of at least a part of said transition section and introducing the withdrawn gases `into 'the furnace in varying amounts in counter relation to movements of the transition zone, the rate of introduction of the withdrawn gases being increased when the transition Zone moves toward the combustion zone, t

said introduction of recirculated furnace gases being effected and controlled from indications of intermediate superheated vapor temperature adjacent the transition zone and from indications of superheated vapor flow, and introducing a controlled proportion of the withdrawn furnace gases into the furnace at a position adjacent the furnace gas outlet, said last introducing of yfurnace gases being controlled from indications of gas temperature at the furnace outlet and from indications ofvapor flow from the unit, theV first mentioned control of recirculated gas introduction including the increase lof recirculated" gas flow as the vapor ilow and intermediate superheated vapor temperature decrease from an optimum value or values while tbe second mentioned control of recirculated gas introduction involves the increase of recirculated `gas flow asV gas temperature at the furnace outlet and rate o `vapor flow tend to increase. l

7. A forced ow once-through vapor `generating and superheating unit comprising walls defining a furnace chamber having a heating gas outlet, means for firing said furnace chamber, vapor generating tubes lining the walls of said furnace chamber and arranged to absorb heat mainlyby radiation from the furnace heating gases, means forming a convection heating gas pass. arranged to receive all of the heating gases from said gas outlet, groups of tubes extendingV across said heating gas pass arranged to absorb heat mainly by convection from said heating gases and serially connected to said furnace wall tubes Vto form at least-one once-through fluid passage arranged to receive a vaporizable liquid at one end andto discharge superheated vapor at the other end, pump means arranged to 615i supply a vaporizable liquid to theinlet end of `said iluid passage, ,said fluid rpassage including a transition point normally located closer to said other end and at which 100% ofthe liquid .willbe vaporized and which tends to move towards the` liquid inlet end of said iluidparssage with a decrease in load on said unit, and means for effectingamovernent ofsaidtransition point awayY from froma-point downstream of said convection heated tubes 'fand Ito introduce the withdrawn gas into said furnace chamber at a position alecting the heat absorption of a major portion of the length of said furnace wall tubes.

8. In a forced ow once-through vapor generating and superheating unit, the method of operation which comprises burning a fuel in a furnace chamber at a location remote from the heating gas outlet therefrom, absorbing heat from the furnace heating gases flowing to the gas oulet mainly by radiation to vapor generating surface lining the boundary walls of the furnace chamber, absorbing heat from all of the heating gases leaving said heating gas outlet mainly by convection to uid heating surface serially connected to the furnace vapor generating surface and forming Vtherewith at least one once-through fluid passage receiving a vaporizable liquid under pressure at one end and discharging superheated vapor at its other end and including a transition point in the convection heated heating surface at which 100% of the liquid will be in a vaporized condition and tending to move toward the liquid inlet end of said fluid passage with a decrease in load on said unit, and effecting a movementrof said transition point away from the liquid inlet end of the fluid passage on a decrease in load by introducing a relatively low temperature gas into the furnace chamber at a position effecting a reduction in the radiant heat absorption of a major portion of the vapor generating surface lining the walls of the furnace chamber.

9. In a forced ow once-through vapor generating and superheating unit, the method of operation which comprises burning a fuel in a furnace chamber at a location remote from the heating gas outlet therefrom, absorbing heat from the furnace heating gases flowing to the gas outlet mainly by radiation to vapor generating surface lining the boundary walls of the furnace chamber,

absorbing heat from all of the heating gases leaving said heating gas outlet mainly by convection to uid heating surface serially connected to the furnace vapor generating surface and forming therewith at least one once-through fluid passage receiving a vaporizable liquid under pressure at one end and discharging superheated vapor at its other end and including a transition point normally located closer to said other end and at which of the liquid will be in a vaporized condition and tending to move toward the liquid inlet end of said fluid passage with a decrease in load on said unit, and effecting a movement of said transition point away from the liquid inlet end of the fluid passage on a decrease in load by withdrawing heating gas from a location downstream of said convection heated heating surface and introducing the withdrawn gas into the furnace chamber at a position effecting a reduction in the radiant heat absorption of a major portion of tne vapor generatingsurface lining the walls of the furnace chamber. 

